1. Field of the Invention
The present invention relates to a four-cycle engine for powering an outboard motor, and in particular, relates to an induction system for such engine.
2. Description of the Related Art
Engines used to power outboard motors are typically water-cooled. The cooling water is usually drawn from the body of water in which the motor is operating through an inlet port in the lower unit of the outboard motor.
The cooling water is pumped from the lower unit to the power head, where it is circulated through various coolant passages formed in the cylinder blocks and cylinder heads to cool the engine. Because the cooling water is drawn from below the engine, it is typically circulated through the coolant passages from the bottom of the engine to the top of the engine. As a result, the bottom cylinders of the engine are generally cooler than the top cylinders.
Fuel tends not to mix with induction air as readily at lower temperatures. As a result, the combustion efficiency of the cooler bottom cylinders is generally lower than the top cylinders. This results in an unbalanced power output between the top and bottom cylinders of the engine.
Induction systems for outboard motors typically include an air intake chamber located within the motor cowling and intake pipes leading from the intake chamber to intake passages formed in the cylinder heads. A throttle valve is usually provided in the intake chamber for regulating the air flow through the intake pipes.
The manufacture and assembly of the intake chamber and intake pipes must be precise so that, during operation of the motor, external air does not leak into the induction system at the junctions between the intake chamber and intake pipes. Because the throttle is located upstream of the junctions between the intake chamber and intake pipes, external air leaking through the junctions can result in unstable engine output.